In the Linux kernel, the following vulnerability has been resolved:

net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

As described in Gerrard's report [1], we have a UAF case when an hfsc class
has a netem child qdisc. The crux of the issue is that hfsc is assuming
that checking for cl->qdisc->q.qlen == 0 guarantees that it hasn't inserted
the class in the vttree or eltree (which is not true for the netem
duplicate case).

This patch checks the n_active class variable to make sure that the code
won't insert the class in the vttree or eltree twice, catering for the
reentrant case.

[1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/

https://git.kernel.org/stable/c/141d34391abbb315d68556b7c67ad97885407547

https://git.kernel.org/stable/c/2e7093c7a8aba5d4f8809f271488e5babe75e202

https://git.kernel.org/stable/c/6082a87af4c52f58150d40dec1716011d871ac21

https://git.kernel.org/stable/c/8df7d37d626430035b413b97cee18396b3450bef

https://git.kernel.org/stable/c/ac39fd4a757584d78ed062d4f6fd913f83bd98b5

https://git.kernel.org/stable/c/e3e949a39a91d1f829a4890e7dfe9417ac72e4d0